1. Field of the Invention
The present invention relates to a method and apparatus for improving performance degradation when channel-coded packet data multiplexed into several bursts is transmitted in a mobile communication system, such as a Time Division Multiple Access (TDMA) communication system or a Code Division Multiple Access (CDMA) communication system.
2. Description of the Related Art
Factors impeding high-speed and high-quality services in wireless communications generally originate from a wireless communication channel environment. For example, a wireless communication channel often experiences changes in the channel environment due to white noise and also signal power changes resulting from fading, shadowing, the Doppler effect according to terminal movements and frequent velocity changes, interference by other users and multipath signals, etc. Thus, in addition to technologies provided in existing 2nd or 3rd generation mobile communication systems, other evolved technologies capable of enhancing adaptability to channel environment changes are required.
In mobile communication systems, channel coding techniques are used to reduce the effect of signal distortion and noise on high-speed data transmission. In 2nd and 3rd generation mobile communication systems, for example, convolutional codes, turbo codes, and the like are widely used as channel encoders. Adaptive Modulation and Coding Schemes (AMCS) and Hybrid Automatic Repeat reQuest (HARQ) are mentioned in both 3rd Generation partnership Project (3GPP) and 3GPP2, which are setting specifications for high-speed data packet transmission systems.
The most widely used 3GPP Global System for Mobile telecommunication/Enhanced Data Rates for GSM Evolution (GSM/EDGE) Radio Access Network (GERAN) system employs a Link Adaptation (LA) technique and an Incremental Redundancy (IR) technique as a link quality control technique.
The LA technique is a method in which Modulation and Coding Schemes (MCS) vary according to channel environment changes. A mobile station measures a Signal-to-Noise Ratio (SNR) and transmits information thereabout to a base station, thereby informing the base station of downlink channel environment. The base station predicts the downlink channel environment based on the information and selects an appropriate MSC based on the predicted value. Thus, in a system using the LA technique, an MCS employing higher order modulation and a high code rate is used for packet data transmission for a terminal neighboring a base station, which usually has good channel environment, and an MCS employing lower order modulation and a low code rate is used for packet data transmission for a terminal with poor channel environment.
The IR is a HARQ technique. When an error occurs in an initially transmitted data packet, packet retransmission is required to compensate for the erroneous packet, and in such a situation, the IR technique is used as a link quality control technique. Technically, the IR technique may be divided into a Full Incremental Redundancy (FIR) technique and a Partial Incremental Redundancy (PIR) technique. The FIR technique improves the performance of a decoder at a receiving end by transmitting a packet including parity bits generated in a channel encoder, instead of transmitting the same packet. That is, the decoder performs decoding by using new parity bits as well as information received at initial transmission, which decreases a code rate, and thus improves the performance of the decoder.
In the 3GPP GERAN system, the GERAN evolution standardization has recently been pursued in order to improve system performance and Quality of Service (QoS). In downlink and uplink packet transmission schemes, higher order modulation (16 QAM, 32 QAM), turbo codes, and an increased symbol rate are newly introduced for high-speed data transmission. Also, a maximum of two Radio Link Control (RLC) data blocks are transmitted per radio block in the existing EDGE system, but the evolved GERAN system enables a maximum of four RLC data blocks to be transmitted per radio block. Thus, a channel coding chain structure for efficient data transmission in an MSC corresponding to a combination of newly designed higher order modulation and turbo codes (or convolutional codes) must be determined.
Additionally, a newly proposed channel coding structure must maintain the same access scheme as that of the existing EDGE system, and ensure backward compatibility, for example, must support a link quality control function.
A Circular Buffer Rate Matching (CBRM) technique is a simple and efficient rate matching technique capable of supporting LA and IR techniques in combination with an MSC level newly added in the GERAN evolution system. Also, the CBRM technique can be used without an external channel interleaver, which reduces the implementation complexity of the system.
FIGS. 1 and 2 illustrate a structure for explaining a CBRM technique of the prior art when an encoder has a mother code rate of ⅓. More specifically, FIG. 1 illustrates an example of a CBRM technique using nonsystematic convolutional codes.
As illustrated in FIG. 1, bits P0, P1, and P2 101 to 103 encoded in a convolutional encoder 100 pass through independent sub-block interleavers 111 to 113, respectively. However, because it is impossible to discern information bits (or systematic bits) from parity bits, when nonsystematic convolutional codes are used, bits 121 to 123 interleaved in the sub-block interleavers 111 to 113 are stored in a Circular Buffer (CB) 130 while being arranged in such a manner as to be interlaced with each other at intervals of one bit.
FIG. 2 illustrates an example of a CBRM technique using turbo codes.
Referring to FIG. 2, information bits and parity bits 201 to 203 pass through independent sub-block interleavers 211 to 213, respectively, in a similar manner as illustrated in FIG. 1. Interleaved bits 221 to 223 are separated into information bits and parity bits, and the separated bits are stored in a CB 230 while being arranged in such a manner as to be interlaced with each other at intervals of one bit, as illustrated in FIG. 1.
An MCS used in the GERAN system requires only two or three rate matching patterns or Redundancy Versions (RVs) to support an IR technique, according to code rates r. That is, two rate matching patterns are needed for a code rate of r≦⅔, and three rate matching patterns are needed for a code rate of r>⅔. Thus, in applying a CBRM technique in the GERAN system, an RV is selected such that transmission data is sequentially selected from a conventional CB. With regard to this, however, data transmitted at retransmission may overlap already transmitted data, resulting in the performance degradation of an IR technique.
In the GERAN system, for example, in GERAN evolution, a minimum of one to four RLC data blocks is transmitted with one radio block. Each RLC data block passes through independent channel coding and puncturing processes, channel interleaving for the whole RLC data to be transmitted is performed, and information distributed on four bursts is transmitted. The GERAN system has a structure in which 8 time slots constitutes one TDMA frame, and one time slot carries one burst. Accordingly, for a terminal not supporting multi-slot capability, four bursts are distributed into and carried by four TDMA frames. Thus, when a CBRM technique is applied in the GERAN system, a CBRM apparatus includes sub-block interleavers, as illustrated in FIGS. 1 and 2, and thus independently channel coded data is generally distributed into and carried by four bursts without passing through an external channel interleaver. Therefore, when several RLC data blocks are sequentially mapped to and carried by four bursts, without passing through an external channel interleaving process, performance degradation may occur due to a burst error.
Particularly, when turbo codes are used, data is separated into information bits and parity bits, and the separated bits are respectively processed so as to maximize coding gain, as illustrated in FIG. 2. Consequently, when several RLC data blocks are sequentially mapped to and carried by four bursts without passing through an external channel interleaving process, performance degradation may occur due to a burst error, as mentioned above.